Postgres 13

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create table rules (id int,   type  text, field text, operator text, criteria text);
insert into rules values 
(1, 'people', 'age', '>', '30'), 
(2, 'people', 'name', '=', 'Alice'),
(2, 'person', 'name', '=', 'Alice');


create table people (id int, name text, age int);
insert into people
values
  (1, 'Emma', 34),
  (2, 'Larry', 25),
  (3, 'Alice', 22),
  (4, 'Thomas', 31)
;

3 rows affected

4 rows affected

create or replace function evaluate_rule(p_row record, p_rule_id int)
  returns boolean
as
$$
declare
  l_expression text;
  l_rule rules;
  l_result boolean;
  l_values jsonb;
  l_type text;
begin
  l_values := to_jsonb(p_row);
  
  select *
    into l_rule
  from rules
  where id = p_rule_id
    and type = pg_typeof(p_row)::text;
  
  if not found then 
    return false;
  end if;
  
  l_type := jsonb_typeof(l_values -> l_rule.field);
  
  if l_type = 'number' then 
    l_expression := format('select %s %s %s', l_values ->> l_rule.field, l_rule.operator, l_rule.criteria);
  else 
    l_expression := format('select %L %s %L', l_values ->> l_rule.field, l_rule.operator, l_rule.criteria);
  end if;
  
  execute l_expression 
    into l_result;
    
  return l_result;
end;
$$
language plpgsql
stable;
select p.*
from people p
where evaluate_rule(p, 1)


id
name
age


1
Emma
34

4
Thomas
31


select p.*
from people p
where evaluate_rule(p, 2)


id
name
age


3
Alice
22


select p.*
from people p
where evaluate_rule(p, 3)